Difficulty has always been encountered when trying to produce prints having the highest quality when they contain both images as well as text and line art. It is well known in the imaging field that to generate the highest quality prints, images (pictorial) need to contain a resolution of 200-300 dpi with 24 bits per color pixel and for text and line art, even higher resolution data is needed, e.g., 500-2000 dpi with 1 bit per pixel.
It is well known that images require a lower resolution, but require many bits per pixel. On the other hand, line art requires high resolution but relatively few bits per pixel. To reduce the cost of a system that handles both high resolution and a large number of bits per pixel, there have been several suggested solutions.
U.S. Pat. No. 5,125,072, issued to Ng in June, 1992, suggests the use of a dual-channel data path. One channel carries the text and line art data at the required high resolution while the second channel carries the image (video) data at the lower resolution, but with a large number of bits per pixel. Each of the two channels have their own storage areas and buffers. The image data is interpolated up (expanded) to equal the resolution of the line art before printing. Then the line art and image data are combined to create the raster lines of the print in a high speed band buffer. The combining process is performed at the highest data rate of the printer and are based on the instructions of a location look-up table.
There are two drawbacks associated with the above teachings. First, such a method and apparatus needs two separate storage areas (frame-stores), with each frame-store requiring its own input and output pipeline along with its own control circuitry. This additional electronics adds considerably to the cost of the entire procedure. It should also be noted that each frame-store must be of substantial size, because any page to be printed may consist of entirely text and line art or images requiring the most memory. The second problem is that the data path comprises many of the functional blocks that must operate at the high clock rate (increased speed) of the print head. Such functional blocks are: the band buffer and the combiner, the location look-up table and its control system, and the print head controller (laser interface). These high speed electronic elements cost more because they are more difficult to develop, manufacture and de-bug.
U.S. Pat. No. 4,135,212 issued to Pugsley et al in January, 1979, is very similar to U.S. Pat. No. 5,125,072, discussed above. The difference is that the '212 patent restricts the resolution of the text and line art area to an integer multiple of the image resolution. Accordingly, the problems discussed above would once again be encountered with the Pugsley et al teachings.
U.S. Pat. No. 5,157,417, issued to Anzai in October, 1992, suggests the use of a printer capable of changing its resolution. The invention describes how the printer resolution could be changed in the vertical direction. There are many problems with such a solution. Firstly, Anzai only teaches how to change the printer resolution in the vertical direction. If the text and images are positioned side by side, this method will not work. Secondly, Anzai teaches the use of a polygon based raster printer that is capable of varying its rotational speed according to the resolution desired. The cost of such a motor controller will be prohibitive when one considers that motor controllers used in present laser printers that rotate at only a single speed are very costly. The reason for such high cost is the requirement for extremely precise speed control that will maintain the velocity variations (flutter) of the polygon at or below 0.1%, motor controllers that provide such low flutter at multiple speeds will certainly cost many times more than present single speed systems. Thirdly, this apparatus is similar to the previous Ng disclosure discussed above and also requires multiple frame-stores for each resolution. As indicated earlier, the extra cost for the duplicate electronic pipelines and extra control circuits reduces the viability of this solution. A fourth problem is encountered when one attempts to vary the clock speed of the laser writer as resolution is being varied. The clock speed of typical laser writers are above 20 MHz and laser driver systems that operate at several different speeds in that range and maintain their precision within the scan line along the printed page will also be too expensive for mass produced printers. A fifth and final problem associated with the Anzai apparatus is the requirement of stopping the printing process whenever the resolution of the printer is changed. The reason for such a delay is to allow the spinning polygon to adjust its speed to accommodate the new resolution. This type of solution cannot be applied to electrophotographic and electrographic printers; because of the different rates of dark decay as one moves along the printed page, any cessation in printing would result in different tone and color reproductions within that page. Furthermore, there is no teaching of how to vary the spot size of the exposing laser beam as the resolution is changed. This has been shown to be possible in some other applications. Once again, the cost would have to increase for a variable spot size laser scanner further impacting the viability of such approach.
U.S. Pat. No. 4,926,200, issued to Ohyama et al in May, 1990 discloses an electrophotographic printer with two exposure heads. One head is designed for exposing images at one resolution and the other head exposes text at another resolution. This invention addresses many of the problems at hand, but would result in prohibitive cost. Exposure subsystems (print heads) are usually one of the most expensive subsystems in the electrophotographic printers. To include two of them in one printer will substantially increase the cost of that printer. Furthermore, one must provide two complete and independent data paths for each print head, clearly increasing the cost of such printer even more.
U.S. Pat. No. 5,045,869, issued to Isaka et al in September, 1991 discloses a printer that can operate at several different resolutions, similar to Anzai's disclosure described above. The difference between Isaka's patent and Anzai's patent is that Anzai claims to change the resolution of the printer on the fly as printing is in progress. Isaka, on the other hand, changes the printer resolution before the printing begins. The selected resolution is to match the needs of the document to be printed. Most of the problems associated with Anzai will find application here as well, such as the use of a dual speed polygon motor and varying the clock speed to change resolution.
Once again, there is no explanation of how to change the spot size of the laser beam as the resolution is changed. One cannot expose with a 300 dpi spot size and expect the equivalent results that a 600 dpi printer would deliver. Similarly, with a spot size properly designed for 600 dpi exposure subsystems, one would not be able to produce solid density patches when it is applied to 300 dpi printing system. The reason is that the spot size for the 600 dpi printer is too small to overlap in two adjacent lines/rows. Therefore, solid areas could not be totally exposed.